1. Field of the Invention
The invention relates to a receiver antenna system with several active antennae.
2. Description of Related Technology
Between the passive antenna structure and the active electronic elements, such as impedance converters and amplifier elements, active receiver antennae do not have interfaces with a constant surge impedance. In the case of passive antennae, the surge impedance of these interfaces must be adapted in the useful frequency range to the surge impedance of a normal line. This therefore reduces the bandwidth of the receiver antenna system as a whole in an undesirable manner.
If an antenna system is formed from several, active individual antennae, of which the respective electrical antenna height is adapted to the respective received frequency range of the individual antenna, in order to avoid deformed antenna patterns—“peaked antenna patterns”—, a broad-band, overall received-frequency range of the receiver antenna system built up from the several individual receiver-frequency ranges of the individual antennae can be achieved. The shortening of the electrical antenna height of the individual antenna can be implemented electrically by arranging impedance elements, for example, a parallel circuit consisting of an inductance and an ohmic resistor at given heights of the individual antenna. At low received frequencies, the inductance bridges the resistor, while the resistor is active at high received frequencies. It is therefore possible to adjust the electrical antenna height to the respective received frequency range of the individual antenna by an exact positioning of the impedance elements and a received-frequency-dependent parametrisation of the impedance elements.
A receiver antenna system including several active individual antennae is disclosed in DE 34 37 727 A1. With the disclosed receiver antenna system, the individual antennae are positioned at relatively large spacing distances—up to a few hundred meters—from one another. The mutual electromagnetic couplings of the individual antennae, which impair the directivity, the efficiency and the antenna power gain of the receiver antenna system, are negligible with an arrangement of this kind. However, if a considerably more compact realization of a receiver antenna system is required with spacing distances between the individual antennae in the order of magnitude of a few centimetres, these mutual, electromagnetic couplings of the individual antennae are no longer negligible. In a disadvantageous manner, they lead to deformed antenna patterns of the individual antennae, to a mutual, negative influence on the base-point impedances and to unsymmetrical stresses on the individual antennae, which has the overall effect of impairing the quality of reception of the receiver antenna system.